FIG. 1 shows a prior art system for the transmission of data by inductive coupling. The system comprises a transceiver station 10 and a transponder type passive module 20, such as an integrated circuit of a contactless chip card. The station 10 comprises an antenna coil 11 forming a resonant circuit 13 with a capacitor 10. The resonant circuit 13 is excited by an AC voltage Vac1 emitted by a generator 14. Facing the station 10, the module 20 has a reception coil 21 forming a resonant circuit 23 with a capacitor 22. This resonant circuit 23 is tuned to the resonant circuit 13 of the station 10.
For the transmission of digital data DT.sub.1,2 from the station 10 to the module 20, the voltage Vac1 is amplitude-modulated by an encoder-modulator circuit 15 receiving the data DT.sub.1,2 to be transmitted. By inductive coupling between the coils 11 and 21, the module 20 receives an induced voltage Vac2 which is a replica of the voltage Vac1, including maximum and minimum values of amplitude. The demodulation of the voltage Vac2 is performed by a demodulator-decoder circuit 25 which provides at an output the data elements DT.sub.1,2.
The transmission of data DT.sub.2,1 in the opposite direction, namely from the module 20 to the station 10, relies on a substantially different method known as a load modulation method. This method includes modulating the impedance of the coil 21, for example, by means of a resistor 26 switched over by a switch 27. The switch 27 is alternately closed and opened by an encoder circuit 28 receiving the data inputs DT.sub.2,1 to be transmitted. The variation of the impedance of the coil 21 is passed on by inductive coupling to the coil 11 of the station 10 and causes modulation of the excitation current or voltage Vac1 in the station 10. A detector-decoder circuit 16 comprises, for example, means for measuring the excitation current or excitation voltage Vac1. The detector-decoder circuit 16 detects the opening and closing of the switch 27 and, from this, determines the data DT.sub.2,1 sent by the module 20.
The various circuits mentioned above are well known to those skilled in the art and shall not be described in detail. We shall now look at the practical approaches of a system of this kind and, more particularly, an approach for station 10. In practice, it is desired to make the station 10 in very compact form with a minimum volume. One approach, as illustrated in FIG. 2, is to arrange the various circuits 14 to 16 and the capacitor 12 on a common interconnection support, e.g., a printed circuit card 17. The antenna coil 11 is also made to be compact and takes the form of flat winding 18 arranged on an insulator support 19 fixed to the printed circuit card 17, e.g., in a perpendicular position.
Although this approach for station 10 is satisfactory in terms of space requirements, the Applicant has realized that positioning of the coil 11 in the vicinity of the electronic circuit appreciably disturbs operation of the station 10. The cause of this disturbance is a parasitic electrical field E emitted by the coil 11 together with the useful magnetic field B (see FIG. 2) which generates parasitic voltages of some microvolts in the conductors. In particular, the electrical field E causes deterioration in the signal-to-noise ratio in a reception mode and disturbs the detection of the very small load modulations when the distance d (see FIG. 1) between the station 10 and the module 20 is great and the inductive coupling between the coils 11, 21 is very low.
One way to overcome this drawback includes placing the printed circuit card 17 in a sheathed metal box, with the coil 11 being placed outside the box. However, this approach has a disadvantage of being costly. Furthermore, this approach leads to an increase in the volume of the station 10, since the coil 11 has to be at a distance from the box so that the magnetic flux B can flow freely without being disturbed by the walls of the box.